Selena GomezMum On BreakupWith Biebs

1/4/2013 5:50 AM PST BY TMZ STAFF

Selena Gomez had nothing to say at LAX about her breakup with Justin Bieber or allegations that the Biebs had been smoking pot.

TMZ broke the story ... Selena and Justin broke up in Mexico on December 30th after a blowout argument. She flew back to L.A. that day and Justin returned on New Year's Eve. As for the nature of the argument, a source tells us, "It's just high school stuff."

The photogs were trying to grill Gomez -- who dressed like a chic Unabomber -- about Justin's alleged marijuana use. A close friend of Chris Guerra -- the photog who was killed on New Year's Day -- told TMZ Guerra﻿ had told himhe saw J.B. driving his Ferrari earlier in the day, smoking pot from a pipe. Sources connected with Bieber insist it's not the case.